Articles, such as absorbent articles, are useful for absorbing many types of fluids, including fluids secreted or eliminated by the human body. Such articles, such as infant diapers, child training pants and adult incontinence products for example, typically comprise an absorbent composite (often as a component referred to as the “core”) which, among other things, performs the function of absorbing and/or retaining fluids which are insulted into an absorbent article. Absorbent composites typically comprise superabsorbent materials in particulate form, as well as cellulose fibers. In general, the superabsorbent material provides fluid absorption capacity while the cellulose fibers function as an aid to enhance fluid wicking and wet integrity, which can allow fuller utilization of the article by allowing fluid to be transported to regions that are away from the point of insult. Thus, while the absorbent composite's liquid retention or storage capacity is due in large part to the superabsorbent particles, the absorbent composite's fibrous matrix provides the essential functions of liquid wicking, intake, distribution, pad strength and integrity, as well as some amount of absorbency under load. These desirable properties are attributable to the fact that the matrix includes cellulosic fibers, typically wood pulp in fiber form, such as cellulose fibers (“fluff”). Cellulose fibers and superabsorbent materials are therefore frequently used in absorbent articles to help improve the absorbent properties of such articles.
There is a continuing effort to improve the performance of such absorbent composites, especially at high levels of fluid saturation, to thereby reduce the occurrence of leakage and to improve fit and comfort. This is particularly significant when such articles are subjected to repeated fluid insults during use. This has become an increasing challenge as recent efforts in absorbent article design have generally focused on using higher concentrations of superabsorbent materials and less fluff to make the absorbent structures thinner and more flexible. However, notwithstanding the increase in total absorbent capacity obtained by increasing the concentration of superabsorbent material, such absorbent articles may still nevertheless leak during use. Such leakage may in part be the result of the absorbent composite component of an absorbent article having an insufficient intake rate (i.e., the rate at which a fluid insult can be taken into and entrained within the absorbent composite for subsequent absorption by the superabsorbent material) due to low permeability and lack of available void volume. Such leakage may in part be the result of the absorbent composite component of an absorbent article having an insufficient utilization efficiency of the entire absorbent composite due to low fluid wicking and low wet integrity.
The inclusion of superabsorbent materials in a fibrous matrix and their incorporation into absorbent composites typically has the effect of reducing an absorbent article's overall bulk while at the same time increasing its liquid absorbent capacity and enhancing skin dryness for the products' wearers. Superabsorbent materials (“superabsorbents”) are generally polymer based and may be available in many forms, such as powders, granules, microparticles and films, for example. Upon contact with fluids, such superabsorbents swell by absorbing the fluids into their structures. In general, superabsorbents are water-swellable, generally water-insoluble absorbent materials having a liquid absorbent capacity of at least about 10, preferably of about 20, and often up to about 100 times their weight in saline or more. In general, superabsorbent materials can quickly absorb fluids insulted into absorbent composites, and can retain such fluids to prevent leakage and help provide a dry feel even after fluid insult.
A variety of materials have been described for use as absorbent materials in absorbent articles. Included among these materials are natural-based materials such as agar, pectin, gums, carboxyalkyl starch and carboxyalkyl cellulosic, such as carboxymethyl cellulose. Natural-based materials tend to form gels rather than maintaining a solid form and are therefore typically not favored in these products. Synthetic materials such as sodium salts of polyacrylates, polyacrylamides, and hydrolyzed polyacrylonitriles have also been used as absorbent materials in absorbent articles. Although natural-based absorbing materials are well known, these materials have not gained wide usage in absorbent articles because of their relatively inferior absorbent properties compared to synthetic absorbent materials, such as sodium polyacrylates. The relatively high cost of these materials has also precluded their use in consumer absorbent products. Furthermore, many natural-based materials tend to form soft, gelatinous masses when swollen with a liquid. The presence of such gelatinous masses in a product's core tends to limit liquid intake, transport and distribution within the core and prevents subsequent liquid insults from being efficiently and effectively absorbed by the product.
In contrast to the natural-based absorbents, synthetic absorbent materials are generally capable of absorbing large quantities of liquid while maintaining a relatively non-gelatinous form. Synthetic absorbent materials, often referred to as superabsorbent polymers (“SAP”), have been incorporated into absorbent composites to provide higher absorbency under pressure and higher absorbency per gram of absorbent material. Superabsorbent polymers are generally supplied as particles having a diameter in the range from about 20-800 microns. Due to their high absorbent capacity under load, absorbent composites that include superabsorbent polymer particles provide the benefit of skin dryness. Because superabsorbent polymer particles can absorb many times their weight in liquid under load, these particles provide the further significant advantages of thinness and wearer comfort. In addition, superabsorbent polymer particles are about half the cost per gram of liquid absorbed under load compared to fluff pulp fibers. For these reasons it is not surprising that there is a growing trend toward higher superabsorbent particle levels and reduced levels of fluff pulp in consumer absorbent products. In fact, some infant diapers for example include 60 to 70 percent by weight (“wt %”) superabsorbent polymer in their liquid storage core. From a cost perspective, an absorbent composite made from 100 wt % superabsorbent particles may be desirable. However, as noted above, such a composite would typically fail to function satisfactorily due to the absence of any significant liquid wicking and distribution of acquired liquid throughout the absorbent composite.
Furthermore, such a composite would tend to lack strength to retain its wet and/or dry structure, shape, and integrity. For example, as superabsorbent content in these products is increased, the absorbent composite suffers in terms of fluid wicking capability as well as wet integrity. When superabsorbent particle content in an absorbent composite is higher than 50 wt %, and particularly as the superabsorbent particle content draws closer to 100 wt %, the absorbent composite exhibits a limited capability of wicking fluid (a function typically performed by fluff) which reduces significantly its utilization efficiency. At the same time, such absorbent composites tend to have almost no wet integrity, particularly after the product is fully loaded with fluid. In order to improve wet integrity of the absorbent composite having high content of superabsorbent particles, binder fiber or adhesive material is often used which can help improve integrity, but at the cost of further reducing fluid wicking capability since the binder fiber and adhesive material are generally hydrophobic in nature. In addition, such materials could add cost to the product.
Another drawback concerning synthetic superabsorbent polymers is their lack of ability to biodegrade. The synthetic polymers' non-biodegradability is disadvantageous with regard to the disposal of used absorbent products containing these polymers.
Cellulosic fibers provide absorbent products with critical functionality that has, to date, not been duplicated by particulate superabsorbent polymers. For absorbent articles comprising absorbent composites, U.S. southern pine fluff pulp is used most often and is often the preferred fiber for such composites. The preference is based on the fluff pulp's advantageous high fiber length (about 2.8 mm) and its relative ease of processing from a wetlaid pulp sheet to an airlaid web. However, these fluff pulp fibers typically only absorb about 2-3 g/g of liquid (e.g., water, saline or bodily fluids) within the fibers' cell walls. Most of the fibers' liquid holding capacity resides in the interstices between fibers. For this reason, a fibrous matrix readily releases acquired liquid on application of pressure. The tendency to release acquired liquid can result in significant skin wetness during use of an absorbent article that includes an absorbent composite formed exclusively from cellulosic fibers. Such articles also tend to leak acquired liquid because liquid is not effectively retained in such a fibrous absorbent composite. This, in turn, reduces product performance as well as confidence by the user.
In some instances, superabsorbent materials have been introduced in synthetic fiber form seeking to provide a material having the functionality of both fiber and superabsorbent polymer particles. However, these superabsorbent fibers are not biodegradable and are difficult to process compared to fluff pulp fibers. In addition, they tend not to blend well with fluff pulp fibers. Furthermore, synthetic superabsorbent fibers are significantly more expensive than superabsorbent polymer particles and, as a result, have not competed effectively for high volume use in absorbent articles.
Attempts have been made to render cellulosic fibers highly absorptive regeneration and by chemical modification to include ionic groups such as carboxylic acid, sulfonic acid, and quaternary ammonium groups that impart water swellability to the fiber. Although some of these modified cellulosic materials are soluble in water, some are water-insoluble. Regardless, none of these highly absorptive modified cellulosic materials possess the structure of a pulp fiber. Rather, these modified cellulosic materials are typically granular or have a regenerated fibril form.
Accordingly, a need exists for a highly absorbent material suitable for use in absorbent articles, where the absorbent material has absorptive properties similar to synthetic, highly absorptive materials and at the same time offers the advantages of liquid wicking and distribution associated with fluff pulp fibers. There is also a need to have an absorbent composite having a superabsorbent content greater than 50 wt %, such as greater than 60 wt %, or greater than 80 wt %, or greater than 90 wt %, or even up to 100 wt %. An absorbent composite having a high superabsorbent content can mean a thinner, lower mass and low cost product. Therefore, there is a further need to develop an absorbent composite which has improved fluid wicking capability while achieving an improved wet integrity for an absorbent composite comprising as much as 100% superabsorbent material.
In addition, there is a need for an absorbent composite comprising a fibrous superabsorbent that combines the advantageous liquid storage capacity provided by superabsorbent polymers and the advantageous liquid wicking and wet integrity provided by fluff pulp fibers. Ideally, the fibrous superabsorbent would be economically viable for use in absorbent articles and would be biodegradable thereby making the disposal of used absorbent products environmentally friendly.